First, nobody is simply tossed into battle. Once a soldier is finished with BCT and AIT, he has to be assigned to a unit. From there he has to be deployed "to the front line" if the Army needs him there.
Now, can he be bumped from the MOS he was promised in his contract? It's very rare but it's happened before. It's all about the "needs of the Army." Hmm... Is there something else going on? Did your brother's friend fail a test? Did a background search turn up some bad news? This kind of thing is more likely.
When civilians sign up, it's true that they can choose their first duty station. The Army does its best to make it happen but sometimes it doesn't work out. It sucks but, again, it's about the needs of the Army.
If your brother's friend believes he was royally screwed, he probably won't reenlist. In the meantime, the soldier has to put national security first. That's the whole point of having an Army.